Corrosion resistance apparatus for use with acid chlorite solutions



'by acid chlorite solutions.

2,910,342 Patented Oct. 27, 195? CORROSION RESISTANCE APPARATUS- FOR USE WITH ACID CI-ILORITESOLUTIONS Deutsche 'Goldu nd Silber-Scheide'an'stalt vormals Roessler, Frankfurt 'am Main, Germany 7 No Drawing. Application September 6, .1956 f L" Serial No. 608,202: F j. Claims priority, application Germany September 10, 1955 V 3 Claims. to]. 21-25 The present invention relates to improvements in ap- Joachim Rasch, Rheinfelden, Germany,. assignor t paratus used in-contact with acid chlorite solutions, such as, for example,acid chlorite bleaching solutions.

It is known that acid chlorine dioxide solutions, such as, for example, areobtained upon acidification of alkali metal and alkaline earth metal chlorites, are rather aggressive and corrode'most materials normally employed I in wooden or ceramic apparatus.

It has also been proposed to add nitrates to the chlorites to be used for bleaching or to add nitric acid or nitrates to the acid chlorite bleaching solutions. However, these proposals did not lead to really satisfactory protection of the apparatus.

While it is known that aluminum is stable against a number of oxidizing substances, it is strongly attacked Also, the thin natural oxide layer which normally forms on aluminum surfaces does not prevent attack by acid chlorite solutions.

It is also known that a very dense and firmly adhering artificial aluminum oxide coating can be achieved .on aluminum by chemical or electrolyte treatment, such as, for example, anodic oxidation. Aluminum provided with anodic oxide coatings is very corrosion resistant in many instances, but is attacked by acid chlorate.

According to the invention, it was unexpectedly discovered that aluminum provided with artificial aluminum oxide coatings obtained by chemical treatment or preferably by anodizing and, if desired, sealed, are corrosion resistant to acid chlorite solutions when care is taken that such solutions are devoid of nitrate and phosphate ions. It was furthermore found that in addition to artificially oxide coated pure aluminum, also artificially coated aluminum containing insignificant quantities of alloying components could be employed. Furthermore,

tions devoid of nitrate and phosphate ions, for example,

The artificially oxidev for bleaching cellulosic materials. coated aluminum not only can beemployed for the bleaching vats, but also parts of the bleaching apparatus, such as pumps, pump parts, spindles for cross-reel bleaching; and the like, which are either in contact with the acid chlorite bleaching solution or are located in the gas space above the bleaching solution. It is, of course, not necessary that the entire apparatus be constructed of the artificially oxide coated aluminum, as naturally the oxide coatedaluminum can be employed as cladding on those parts which are'subject to corrosion duringthe bleach operation, it merely being necessary that an artificially oxidized aluminumsurface on the aluminum be presented to the corrosive influences. J H

The following table illustrates the exceptionally good corrosion resistance of anodized aluminum to acid chlorite solutions in the absence of nitrate and phosphate ions.

In the tests, chlorite solutions containing 1 g. per liter of sodium chlorite were employed and the tests were carried out at 70 C. In such tests, anodized aluminum strips were immersed in the chloritesolution so that of the length thereof was'below the surface of the solution and, above. The chlorite solutions employed were acidified to a pH of 3.8 withv the acid indicated in the table.

1 Table Change in weight, gJmfl/day Aluminum base Acid and other additions After After After 5 days 10 days 15 days Al, 99.99% HCOOH +0. 030 0. 048 0. 060 CHQCOOH +0046 +0.015 +0023 HCOOH+NAClOa- +0050 ;|;0.000 d:0.000 HNO +0. 119 +0. 059 0. 719 HaPO4 0. 059 0. 100 0. 105 HCOOH+Na4PzO7. 0.882 0. 778 0. 756 HO%+NaNO; 2.060 1. 485 0. 988

The anodic oxide coating on the duced as follows: 1

A degreased aluminum strip (99%) is thoroughly pur'i fied by a short immersion into a solution containing about 20% of sodium hydroxide at a temperature of C. whereby the bath is vehemently foaming. The aluminum strip is then rinsed with water and pickled with a cold 3% nitric acid whereupon a second rinsing with water follows. After this purification process the aluminum is further treated in accordance with the examples:

(1) Electrolytic 0xidati0n.1nto a bath of cold chemically pure 20% sulfuric acid a cathode of graphite or aluminum is suspended whilst the working piece to be oxidized is suspended as anode. After applying a voltage of about 15 volts or more a current density of about 0.3 to 1.5 amp./dm. is obtained. After a period of /2 to 2 hours the electrolysis is switched oflF, the working piece removed and thoroughly rinsed for half an hour.

To harden the oxide film the oxidized aluminum is treated with boiling water or a diluted solution of sodium silicate for a period of /z to 2 hours.

Instead of sulfuric acid (20%) a 10% oxalic acid, combined with chromicacid (for instance, 0.1% CrO if desired; or pure chromic acid 3 to 10%, may be employed accordingly.

(2) Chemical,0xidati0n.-For a period of 12 to 20 minutes an aluminum strip is immersed in a solution containing 50 grs./liter of soda and 15 grs./liter of sodium aluminum was proof 90 to 100 c. After recirculation blea'ching apparatus made of the thus coated aluminum in a bleaching bath consisting of 9 m3 of water and 30 kgs. of sodium chlorite 80% (the. chlorite must be free of nitrates which are sometimes added as protective agents against corrosion; in general, the chlorites with such an addition of nitrate, however, have only a content of NaClO of 50%); PH alue adjusted to 3.9 to 4.1 by means of acetic acid. The treatment is carried out during a period of 4 to 5 hours and atv a temperature of 60 to 100 C. v I

(4) To the bleaching bath. as described in the foregoing example to 20 kgs. of sodium or potassium chlorate are added. Adjustment of the pH value (3.7 to 3.8) may be carried out with formic acid or acetic acid either; in the latter case, however, the pH value should be 3.9 to 4.1.

A careful subsequent examination of the bleaching apparatus used in the aforementioned tests revealed no corrosion leading to a destruction of the apparatus and no trouble in the bleaching process.

I claim:

1. A method of bleaching cellulosic materials which Q comprises bleaching such cellulosic materials with acid chlorite bleaching solutions devoid of nitrate and phosphate ions in an apparatus in which at least a portion of the surfaces in contact with the bleaching solution and the gas space above the bleaching solution are anodically oxidized aluminum surfaces.

2,.An acid. chlorite bleaching solution confined in; a container in which at least a portion of the surfaces in contact with such bleaching solution and the gas space above such bleaching solution are constructed of anodically oxide coated aluminum, said bleaching solution I being devoid of nitrate and phosphate ions.

3. An acid'chlorite bleaching solution confined in a container in which'at least a portion of the surfaces in contact with such bleaching solution and the gas space OTHER REFERENCES Cheng:.Metall.,'vo1. 8, Nos. 5/6, March 1954, pp. 193, l9.4.l99,TN'3,.M38.

Taylor: Technical Asso. Papers, Ser. XXIII, 1940, pp. 251-256. 

1. A METHOD OF BLEACHING CELLULOSE MATERIALS WHICH COMPRISES BLEACHING SUCH CELLULOSIC MATERIALS WITH ACID CHLORIDE BLEACHING SOLUTIONS DEVOID OF NITRATE AND PHOSPHATE IONS IN AN APPARATUS IN WHICH AT LEAST A PORTION OF THE SURFACES IN CONTACT WITH THE BLEACHING SOLUTION AND THE GAS SPACE ABOVE THE BLEACHING SOLUTION ARE ANODICALLY OXIDIZED ALUMINUM SURFACES. 